Rickettsia species identified in adult, host-seeking Dermacentor occidentalis (Acari: Ixodidae) from Baja California, Mexico, and Oregon and Washington, United States.

The Pacific Coast tick (Dermacentor occidentalis Marx, 1892) is a frequently encountered and commonly reported human-biting tick species that has been recorded from most of California and parts of southwestern Oregon, southcentral Washington, and northwestern Mexico. Although previous investigators have surveyed populations of D. occidentalis for the presence of Rickettsia species across several regions of California, populations of this tick have not been surveyed heretofore for rickettsiae from Baja California, Oregon, or Washington. We evaluated 1,367 host-seeking, D. occidentalis adults collected from 2015 to 2022 by flagging vegetation at multiple sites in Baja California, Mexico, and Oregon and Washington, United States, using genus- and species-specific assays for spotted fever group rickettsiae. DNA of Rickettsia 364D, R. bellii, and R. tillamookensis was not detected in specimens from these regions. DNA of R. rhipicephali was detected in D. occidentalis specimens obtained from Ensenada Municipality in Baja California and southwestern Oregon, but not from Washington. All ompA sequences of R. rhipichephali that were amplified from individual ticks in southwestern Oregon were represented by a single genotype. DNA of the Ixodes pacificus rickettsial endosymbiont was amplified from specimens collected in southwestern Oregon and Klickitat County, Washington; to the best of our knowledge, this Rickettsia species has never been identified in D. occidentalis. Collectively, these data are consistent with a relatively recent introduction of Pacific Coast ticks in the northernmost extension of its recognized range.

Recovery of western black-legged tick and vertebrate populations after a destructive wildfire in an intensively-studied woodland in northern California.

Despite increasing severity and frequency of wildfires, knowledge about how fire impacts the ecology of tick-borne pathogens is limited. In 2018, the River Fire burned a forest in the far-western U.S.A. where the ecology of tick-borne pathogens had been studied for decades. Forest structure, avifauna, large and small mammals, lizards, ticks, and tick-borne pathogens (Anaplasma phagocytophilum, Borrelia burgdorferi, Borrelia miyamotoi) were assessed after the wildfire in 2019 and 2020. Burning reduced canopy cover and eliminated the layer of thick leaf litter that hosted free-living ticks, which over time was replaced by forbs and grasses. Tick abundance and the vertebrate host community changed dramatically. Avian species adapted to cavity nesting became most prevalent, while the number of foliage-foraging species increased by 83% as vegetation regenerated. Nine mammalian species were observed on camera traps, including sentinel (black-tailed jackrabbits) and reservoir hosts (western gray squirrels) of B. burgdorferi. One Peromyscus sp. mouse was captured in 2019 but by 2020, numbers were rebounding (n=37), although tick infestations on rodents remained sparse (0.2/rodent). However, western fence lizards (n=19) hosted 8.6 ticks on average in 2020. Assays for pathogens found no B. miyamotoi in either questing or host-feeding ticks, A. phagocytophilum DNA in 4% (1/23) in 2019, and 17% (29/173) in 2020 for questing and host-feeding ticks combined, and B. burgdorferi DNA in just 1% of all ticks collected in 2020 (2/173). We conclude that a moderately severe wildfire can have dramatic impacts on the ecology of tick-borne pathogens, with changes posited to continue for multiple years.

Detection and Isolation of Rickettsia tillamookensis (Rickettsiales: Rickettsiaceae) From Ixodes pacificus (Acari: Ixodidae) From Multiple Regions of California.

The western black-legged tick (Ixodes pacificus) is the most frequently identified human-biting tick species in the western United States and the principal vector of at least three recognized bacterial pathogens of humans. A potentially pathogenic Rickettsia species, first described in 1978 and recently characterized as a novel transitional group agent designated as Rickettsia tillamookensis, also exists among populations of I. pacificus, although the distribution and frequency of this agent are poorly known. We evaluated DNA extracts from 348 host-seeking I. pacificus nymphs collected from 9 locations in five California counties, and from 916 I. pacificus adults collected from 24 locations in 13 counties, by using a real-time PCR designed specifically to detect DNA of R. tillamookensis. DNA of R. tillamookensis was detected in 10 (2.9%) nymphs (95% CI: 1.6-5.2%) and 17 (1.9%) adults (95% CI: 1.2-3.0%) from 11 counties of northern California. Although site-specific infection rates varied greatly, frequencies of infection remained consistently low when aggregated by stage, sex, habitat type, or geographical region. Four novel isolates of R. tillamookensis were cultivated in Vero E6 cells from individual adult ticks collected from Alameda, Nevada, and Yolo counties. Four historical isolates, serotyped previously as 'Tillamook-like' strains over 40 yr ago, were revived from long-term storage in liquid nitrogen and confirmed subsequently by molecular methods as isolates of R. tillamookensis. The potential public health impact of R. tillamookensis requires further investigation.

Prevalence of Babesia canis DNA in Ixodes ricinus ticks collected in forest and urban ecosystems in west-central Poland.

Babesia canis, a widely distributed European tick-borne protozoan haemoparasite, causes canine babesiosis, the most important tick-borne disease afflicting dogs worldwide. The meadow tick, Dermacentor reticulatus, is considered to be the primary vector of this parasite in central Europe. Females of the more broadly distributed and medically important castor bean tick, Ixodes ricinus, also commonly feed upon dogs, but their role in the enzootic transmission cycle of B. canis is unclear. Here, we screened 1,598 host-seeking I. ricinus ticks collected from two different ecosystems, forest stands vs. urban recreational forests, for the presence of B. canis DNA. Ticks were sampled during their two seasonal peaks of activity, spring (May/June) and late summer (September). Babesia species were identified by amplification and sequencing of a hypervariable 18S rRNA gene fragment. Babesia canis was the only piroplasm detected in 13% of 200 larvae and 8.2% of 324 nymphs in the forest ecosystems. In urban recreational areas, B. canis DNA was found in 1.5% of 460 nymphs, 3.5% of 289 females and 3.2% of 280 males. Additionally, three samples, including one female, one male, and one nymph, were co-infected with B. venatorum and one nymph with B. divergens or B. capreoli. Our findings implicate that B. canis can be transmitted transovarially and maintained transstadially within populations of I. ricinus, but the vector competence of I. ricinus for transmitting B. canis remains to be investigated.

Modeling future climate suitability for the western blacklegged tick, Ixodes pacificus, in California with an emphasis on land access and ownership.

In the western United States, Ixodes pacificus Cooley & Kohls (Acari: Ixodidae) is the primary vector of the agents causing Lyme disease and granulocytic anaplasmosis in humans. The geographic distribution of the tick is associated with climatic variables that include temperature, precipitation, and humidity, and biotic factors such as the spatial distribution of its primary vertebrate hosts. Here, we explore (1) how climate change may alter the geographic distribution of I. pacificus in California, USA, during the 21st century, and (2) the spatial overlap among predicted changes in tick habitat suitability, land access, and ownership. Maps of potential future suitability for I. pacificus were generated by applying climate-based species distribution models to a multi-model ensemble of climate change projections for the Representative Concentration Pathway (RCP) 4.5 (moderate emission) and 8.5 (high emission) scenarios for two future periods: mid-century (2026-2045) and end-of-century (2086-2099). Areas climatically-suitable for I. pacificus are projected to expand by 23% (mid-century RCP 4.5) to 86% (end-of-century RCP 8.5) across California, compared to the historical period (1980-2014), with future estimates of total suitable land area ranging from about 88 to 133 thousand km2, or up to about a third of California. Regions projected to have the largest area increases in suitability by end-of-century are in northwestern California and the south central and southern coastal ranges. Over a third of the future suitable habitat is on lands currently designated as open access (i.e. publicly available), and by 2100, the amount of these lands that are suitable habitat for I. pacificus is projected to more than double under the most extreme emissions scenario (from ~23,000 to >51,000 km2). Of this area, most is federally-owned (>45,000 km2). By the end of the century, 26% of all federal land in the state is predicted to be suitable habitat for I. pacificus. The resulting maps may facilitate regional planning and preparedness by informing public health and vector control decision-makers.

Borrelia maritima sp. nov., a novel species of the Borrelia burgdorferi sensu lato complex, occupying a basal position to North American species.

Borrelia species are vector-borne parasitic bacteria with unusual, highly fragmented genomes that include a linear chromosome and linear as well as circular plasmids that differ numerically between and within various species. Strain CA690T, which was cultivated from a questing Ixodes spinipalpis nymph in the San Francisco Bay area, CA, was determined to be genetically distinct from all other described species belonging to the Borrelia burgdorferi sensu lato complex. The genome, including plasmids, was assembled using a hybrid assembly of short Illumina reads and long reads obtained via Oxford Nanopore Technology. We found that strain CA690T has a main linear chromosome containing 902176 bp with a blast identity ≤91 % compared with other Borrelia species chromosomes and five linear and two circular plasmids. A phylogeny based on 37 single-copy genes of the main linear chromosome and rooted with the relapsing fever species Borrelia duttonii strain Ly revealed that strain CA690T had a sister-group relationship with, and occupied a basal position to, species occurring in North America. We propose to name this species Borrelia maritima sp. nov. The type strain, CA690T, has been deposited in two national culture collections, DSMZ (=107169) and ATCC (=TSD-160).

Phylogeography of Borrelia spirochetes in Ixodes pacificus and Ixodes spinipalpis ticks highlights differential acarological risk of tick-borne disease transmission in northern versus southern California.

The common human-biting tick, Ixodes pacificus, is the primary vector of the Lyme disease spirochete, Borrelia burgdorferi sensu stricto (ss) in western North America and has been found to harbor other closely-related spirochetes in the Borrelia burgdorferi sensu lato (sl) complex. Between 2008-2015, 11,066 adult and 3,815 nymphal I. pacificus and five adult and 144 nymphal Ixodes spinpalpis, a commonly collected wildlife tick, were collected from 42 California counties. Borrelia burgdorferi sl was detected in 1.2% and 3.8% I. pacificus adults and nymphs, respectively. Results from this study indicate genetic diversity and geographic structure of B. burgdorferi sl in California I. pacificus ticks, by sequence comparison of the16S rRNA gene, with B. burgdorferi ss, the agent of Lyme disease, found only in I. pacificus collected from the north and central coastal and Sierra Nevada foothill regions; B. burgdorferi ss was not detected in ticks tested from southern California. In contrast, Borrelia bissettiae, a member of the B. burgdorferi sl complex, was detected in both I. pacificus and I. spinipalpis, in the coastal region of both northern and southern California, but was absent from ticks in the Sierra Nevada foothills. In a similar pattern to B. bissettiae, Borrelia americana (a member of the B. burgdorferi sl complex) was detected in a single adult I. pacificus from the north coast and two I. spinipalpis nymphs from south-coastal California. This study highlights that the geographic area of Lyme disease acarological risk in California is the north-central and Sierra Nevada foothill regions of the state with little to no risk in the southern regions of the state.

Evidence for transmission of the zoonotic apicomplexan parasite Babesia duncani by the tick Dermacentor albipictus.

Babesiosis is a potentially fatal tick-borne zoonotic disease caused by a species complex of blood parasites that can infect a variety of vertebrates, particularly dogs, cattle, and humans. In the United States, human babesiosis is caused by two distinct parasites, Babesia microti and Babesia duncani. The enzootic cycle of B. microti, endemic in the northeastern and upper midwestern regions, has been well characterised. In the western United States, however, the natural reservoir host and tick vector have not been identified for B. duncani, greatly impeding efforts to understand and manage this zoonotic disease. Two and a half decades after B. duncani was first described in a human patient in Washington State, USA, we provide evidence that the enzootic tick vector is the winter tick, Dermacentor albipictus, and the reservoir host is likely the mule deer, Odocoileus hemionus. The broad, overlapping ranges of these two species covers a large portion of far-western North America, and is consistent with confirmed cases of B. duncani in the far-western United States.

Rickettsia Species Isolated from Dermacentor occidentalis (Acari: Ixodidae) from California.

The Pacific Coast tick (Dermacentor occidentalis Marx, 1892) is one of the most widely distributed and frequently encountered tick species in California. This tick is the primary vector of an unclassified spotted fever group rickettsial pathogen, designated currently as Rickettsia 364D, the etiologic agent of a recently recognized tick-borne rickettsiosis known as Pacific Coast tick fever. Despite intensified interest in this pathogen, important questions remain regarding its taxonomic status and possible variations in genotype among different strains that could influence its pathogenicity. Only the extensively passaged prototypical isolate (strain 364-D) is widely available to rickettsiologists and public health scientists worldwide. To achieve a larger, more geographically diverse, and contemporary collection of strains, 1,060 questing adult D. occidentalis ticks were collected from 18 sites across six counties in northern and southern California in 2016 and 2017. Fourteen ticks (1.3%) yielded DNA of Rickettsia 364D and from these, 10 unique isolates from Lake and Orange counties were obtained. Additionally, Rickettsia rhipicephali was detected in 108 (10.2%) ticks, from which eight isolates were obtained, and Rickettsia bellii in six (0.6%), from which three isolates were obtained. The panel of recently acquired, low-passage strains of Rickettsia 364D derived from this study could enhance opportunities for investigators to accurately determine the taxonomic standing of this agent and to develop specific diagnostic assays for detecting infections with Rickettsia 364D in ticks and humans.

Modeling Climate Suitability of the Western Blacklegged Tick in California.

Ixodes pacificus Cooley & Kohls (Acari: Ixodidae), the primary vector of Lyme disease spirochetes to humans in the far-western United States, is broadly distributed across Pacific Coast states, but its distribution is not uniform within this large, ecologically diverse region. To identify areas of suitable habitat, we assembled records of locations throughout California where two or more I. pacificus were collected from vegetation from 1980 to 2014. We then employed ensemble species distribution modeling to identify suitable climatic conditions for the tick and restricted the results to land cover classes where these ticks are typically encountered (i.e., forest, grass, scrub-shrub, riparian). Cold-season temperature and rainfall are particularly important abiotic drivers of suitability, explaining between 50 and 99% of the spatial variability across California among models. The likelihood of an area being classified as suitable increases steadily with increasing temperatures >0°C during the coldest quarter of the year, and further increases when precipitation amounts range from 400 to 800 mm during the coldest quarter, indicating that areas in California with relatively warm and wet winters typically are most suitable for I. pacificus. Other consistent predictors of suitability include increasing autumn humidity, temperatures in the warmest month between 23 and 33°C, and low-temperature variability throughout the year. The resultant climatic suitability maps indicate that coastal California, especially the northern coast, and the western Sierra Nevada foothills have the highest probability of I. pacificus presence.

Spatial clustering of Borrelia burgdorferi sensu lato within populations of Allen's chipmunks and dusky-footed woodrats in northwestern California.

The ecology of Lyme borreliosis is complex in northwestern California, with several potential reservoir hosts, tick vectors, and genospecies of Borrelia burgdorferi sensu lato. The primary objective of this study was to determine the fine-scale spatial distribution of different genospecies in four rodent species, the California ground squirrel (Otospermophilus beecheyi), northern flying squirrel (Glaucomys sabrinus), dusky-footed woodrat (Neotoma fuscipes), and Allen's chipmunk (Neotamias senex). Rodents were live-trapped between June 2004 and May 2005 at the Hoopa Valley Tribal Reservation (HVTR) in Humboldt County, California. Ear-punch biopsies obtained from each rodent were tested by polymerase chain reaction (PCR) and sequencing analysis. The programs ArcGIS and SaTScan were used to examine the spatial distribution of genospecies. Multinomial log-linear models were used to model habitat and host-specific characteristics and their effect on the presence of each borrelial genospecies. The Akaike information criterion (AICc) was used to compare models and determine model fit. Borrelia burgdorferi sensu stricto was primarily associated with chipmunks and B. bissettiae largely with woodrats. The top model included the variables "host species", "month", and "elevation" (weight = 0.84). Spatial clustering of B. bissettiae was detected in the northwestern section of the HVTR, whereas B. burgdorferi sensu stricto was clustered in the southeastern section. We conclude that the spatial distribution of these borreliae are driven at least in part by host species, time-of-year, and elevation.

Human Borrelia miyamotoi infection in California: Serodiagnosis is complicated by multiple endemic Borrelia species.

To determine whether human Borrelia miyamotoi infection occurs in the far-western United States, we tested archived sera from northwestern California residents for antibodies to this emerging relapsing fever spirochete. These residents frequently were exposed to I. pacificus ticks in a region where B. miyamotoi tick infection has been reported. We used a two-step B. miyamotoi rGlpQ assay and a B. miyamotoi whole-cell lysate (WCL) assay to detect B. miyamotoi antibody. We also employed Borrelia hermsii and Borrelia burgdorferi WCL assays to examine if these Borrelia induce cross reacting antibody to B. miyamotoi. Sera were collected from 101 residents in each of two consecutive years. The sera of 12 and 14 residents in years one and two, respectively, were B. miyamotoi rGlpQ seroreactive. Sufficient sera were available to test 15 of the 26 seropositive samples using B. miyamotoi and B. hermsii WCL assays. Two residents in year one and seven residents in year two were seroreactive to both Borrelia antigens. Although discernible differences in seroreactivity were evident between the B. miyamotoi and B. hermsii WCL assays, infection with one or the other could not be determined with certainty. Sera from two Borrelia burgdorferi /B. miyamotoi seropositive subjects reacted strongly against B. miyamotoi and B. hermsii WCL antigens. Ecological, epidemiological, and clinical data implicated B. miyamotoi as the probable cause of infection among those whose sera reacted against both antigens. Our findings suggest that human B. miyamotoi infection occurs in northern California and that B. hermsii and B. burgdorferi infections produce antibodies that cross-react with B. miyamotoi antigens. Health care professionals in the far-western United States should be aware that B. miyamotoi disease may occur throughout the geographic distribution of I. pacificus and that improved relapsing fever group spirochete antibody assays are urgently needed.

Prevalence and Geographic Distribution of Borrelia miyamotoi in Host-Seeking Ixodes pacificus (Acari: Ixodidae) Nymphs in Mendocino County, California.

Borrelia miyamotoi is an increasingly recognized human pathogen transmitted by Ixodes ticks in the Northern Hemisphere. In North America, infection prevalences of B. miyamotoi are characteristically low (<10%) in Ixodes scapularis (Say; Acari: Ixodidae) and Ixodes pacificus (Cooley & Kohls; Acari: Ixodidae), both of which readily bite humans. We tested 3,255 host-seeking I. pacificus nymphs collected in 2004 from 79 sites throughout Mendocino County in north-coastal California for presence of B. miyamotoi. The collection sites represented a variety of forest types ranging from hot, dry oak woodlands in the southeast, to coastal redwoods in the west, and Ponderosa pine and Douglas fir-dominated areas in the northern part of the county. We found that B. miyamotoi was geographically widespread, but infected I. pacificus nymphs infrequently (cumulative prevalence of 1.4%). Infection prevalence was not significantly associated with geographic region or woodland type, and neither density of host-seeking nymphs, nor infection with Borrelia burgdorferi sensu stricto was associated with B. miyamotoi infection status in individual ticks. Because B. burgdorferi prevalence at the same sites was previously associated with woodland type and nymphal density, our results suggest that despite sharing a common vector, the primary modes of enzootic maintenance for the two pathogens are likely different.

Metagenomic-based Surveillance of Pacific Coast tick Dermacentor occidentalis Identifies Two Novel Bunyaviruses and an Emerging Human Ricksettsial Pathogen.

An increasing number of emerging tick-borne diseases has been reported in the United States since the 1970s. Using metagenomic next generation sequencing, we detected nucleic acid sequences from 2 novel viruses in the family Bunyaviridae and an emerging human rickettsial pathogen, Rickettsia philipii, in a population of the Pacific Coast tick, Dermacentor occidentalis in Mendocino County sampled annually from 2011 to 2014. A total of 250 adults of this human-biting, generalist tick were collected from contiguous chaparral and grassland habitats, and RNA from each individually extracted tick was deep sequenced to an average depth of 7.3 million reads. We detected a Francisella endosymbiont in 174 ticks (70%), and Rickettsia spp. in 19 ticks (8%); Rickettsia-infected ticks contained R. rhipicephali (16 of 250, 6.4%) or R. philipii (3 of 250,1.2%), the agent of eschar-associated febrile illness in humans. The genomes of 2 novel bunyaviruses (>99% complete) in the genera Nairovirus and Phlebovirus were also identified and found to be present in 20-91% of ticks, depending on the year of collection. The high prevalence of these bunyaviruses in sampled Dermacentor ticks suggests that they may be viral endosymbionts, although further studies are needed to determine whether they are infectious for vertebrate hosts, especially humans, and their potential role in tick ecology.

A Molecular Survey for Francisella tularensis and Rickettsia spp. in Haemaphysalis leporispalustris (Acari: Ixodidae) in Northern California.

Francisella tularensis and Rickettsia spp. have been cultured from Haemaphysalis leporispalustris Packard, but their prevalence in this tick has not been determined using modern molecular methods. We collected H. leporispalustris by flagging vegetation and leaf litter and from lagomorphs (Lepus californicus Gray and Sylvilagus bachmani (Waterhouse)) in northern California. Francisella tularensis DNA was not detected in any of 1,030 ticks tested by polymerase chain reaction (PCR), whereas 0.4% of larvae tested in pools, 0 of 117 individual nymphs, and 2.3% of 164 adult ticks were PCR-positive for Rickettsia spp. Positive sites were Laurel Canyon Trail in Tilden Regional Park in Alameda Contra Costa County, with a Rickettsia spp. prevalence of 0.6% in 2009, and Hopland Research and Extension Center in Mendocino County, with a prevalence of 4.2% in 1988. DNA sequencing revealed R. felis, the agent of cat-flea typhus, in two larval pools from shaded California bay and live oak leaf litter in Contra Costa County and one adult tick from a L. californicus in chaparral in Mendocino County. The R. felis in unfed, questing larvae demonstrates that H. leporispalustris can transmit this rickettsia transovarially. Although R. felis is increasingly found in diverse arthropods and geographical regions, prior literature suggests a typical epidemiological cycle involving mesocarnivores and the cat flea, Ctenocephalides felis. To our knowledge, this is the first report of R. felis in H. leporispalustris. Natural infection and transovarial transmission of this pathogen in the tick indicate the existence of a previously undocumented wild-lands transmission cycle that may intersect mesocarnivore-reservoired cycles and collectively affect human health risk.

The Eco-epidemiology of Pacific Coast Tick Fever in California.

Rickettsia philipii (type strain "Rickettsia 364D"), the etiologic agent of Pacific Coast tick fever (PCTF), is transmitted to people by the Pacific Coast tick, Dermacentor occidentalis. Following the first confirmed human case of PCTF in 2008, 13 additional human cases have been reported in California, more than half of which were pediatric cases. The most common features of PCTF are the presence of at least one necrotic lesion known as an eschar (100%), fever (85%), and headache (79%); four case-patients required hospitalization and four had multiple eschars. Findings presented here implicate the nymphal or larval stages of D. occidentalis as the primary vectors of R. philipii to people. Peak transmission risk from ticks to people occurs in late summer. Rickettsia philipii DNA was detected in D. occidentalis ticks from 15 of 37 California counties. Similarly, non-pathogenic Rickettsia rhipicephali DNA was detected in D. occidentalis in 29 of 38 counties with an average prevalence of 12.0% in adult ticks. In total, 5,601 ticks tested from 2009 through 2015 yielded an overall R. philipii infection prevalence of 2.1% in adults, 0.9% in nymphs and a minimum infection prevalence of 0.4% in larval pools. Although most human cases of PCTF have been reported from northern California, acarological surveillance suggests that R. philipii may occur throughout the distribution range of D. occidentalis.

Recurrent evolution of host and vector association in bacteria of the Borrelia burgdorferi sensu lato species complex.

BACKGROUND: The Borrelia burgdorferi sensu lato (s.l.) species complex consists of tick-transmitted bacteria and currently comprises approximately 20 named and proposed genospecies some of which are known to cause Lyme Borreliosis. Species have been defined via genetic distances and ecological niches they occupy. Understanding the evolutionary relationship of species of the complex is fundamental to explaining patterns of speciation. This in turn forms a crucial basis to frame testable hypotheses concerning the underlying processes including host and vector adaptations. RESULTS: Illumina Technology was used to obtain genome-wide sequence data for 93 strains of 14 named genospecies of the B. burgdorferi species complex and genomic data already published for 18 additional strain (including one new species) was added. Phylogenetic reconstruction based on 114 orthologous single copy genes shows that the genospecies represent clearly distinguishable taxa with recent and still ongoing speciation events apparent in Europe and Asia. The position of Borrelia species in the phylogeny is consistent with host associations constituting a major driver for speciation. Interestingly, the data also demonstrate that vector associations are an additional driver for diversification in this tick-borne species complex. This is particularly obvious in B. bavariensis, a rodent adapted species that has diverged from the bird-associated B. garinii most likely in Asia. It now consists of two populations one of which most probably invaded Europe following adaptation to a new vector (Ixodes ricinus) and currently expands its distribution range. CONCLUSIONS: The results imply that genotypes/species with novel properties regarding host or vector associations have evolved recurrently during the history of the species complex and may emerge at any time. We suggest that the finding of vector associations as a driver for diversification may be a general pattern for tick-borne pathogens. The core genome analysis presented here provides an important source for investigations of the underlying mechanisms of speciation in tick-borne pathogens.

Human behaviors elevating the risk of exposure to Ixodes ricinus larvae and nymphs in two types of lowland coniferous forests in west-central Poland.

Ixodes ricinus L. is the commonest tick encountered by humans in Central Europe and the most important vector of tick-borne diseases (TBD) in Europe. Foresters represent a group at elevated risk of exposure to I. ricinus throughout Europe. Here, we employed an experimental design similar to one used previously to determine what behavioral factors heighten exposure to questing Ixodes pacificus nymphs in a hardwood forest in the far-western United States. Five activities were evaluated systematically in a coniferous forest and a mixed broadleaved forest in west-central Poland: (i) walking in low-cut running shoes; (ii) sitting atop logs; (iii) sitting on leaf litter; (iv) sitting against tree trunks; and (v) gathering firewood. In total, 540 I. ricinus ticks were collected (198 larvae, 327 nymphs, 15 adults) in spring and late summer. Many more ticks were collected in spring than in late summer, and in the mixed broadleaved forest versus the coniferous forest. The riskiest behavior for acquiring nymphs in both spring and late summer was gathering firewood, though sitting atop logs was nearly as perilous in late summer. In contrast, the riskiest behaviors for encountering larvae in spring and late summer were sitting against trunks and sitting on leaf litter, respectively. Pairwise comparisons revealed only two significant associations among the variables explored for the nymphs: season and human gender, and season and type of forest. For larval ticks, the most significant associations were gender and sitting behaviors and for type of forest and sitting on leaf litter or against tree trunks. The most commonly infested body region was the lower legs.

Diapause in ticks of the medically important Ixodes ricinus species complex.

Four members of the Ixodes ricinus species complex, Ixodes pacificus, Ixodes persulcatus, Ixodes ricinus and Ixodes scapularis, have, between them, a worldwide distribution within the northern hemisphere. They are responsible for the transmission of several animal and human pathogens, including the causal agents of Lyme borreliosis, tick-borne encephalitis, human granulocytic anaplasmosis and human babesiosis. Despite the importance of these ticks as vectors, the knowledge and understanding of the role that diapause plays in their complex life cycles are confused and incomplete. In view of the continuing geographic spread of these tick species, as well as the effects of climate change on vector-borne diseases, it is timely to encourage research on diapause phenomena to improve understanding of their biology and of pathogen transmission dynamics. In our review we seek to clarify thinking on the topic and to address gaps in our knowledge that require the attention of researchers.